Recently, as portable electronic devices have been widely distributed and have become smaller, thinner, and lighter in weight, studies have been actively conducted to make a secondary battery used as a power source thereof small and lightweight while being chargeable and dischargeable for a long time.
The lithium secondary battery generates electrical energy by oxidation and reduction reactions when lithium ions are inserted into and removed from a cathode and an anode, and is manufactured by using materials capable of inserting and removing lithium ions as the cathode and the anode, and filling an organic electrolyte or a polymer electrolyte between the cathode and the anode.
The organic electrolyte that is currently widely used may include ethylene carbonate, propylene carbonate, dimethoxyethane, gamma butyrolactone, N,N-dimethylformamide, tetrahydrofuran, acetonitrile, etc. However, the organic electrolyte is generally easy to be volatile and has high flammability, and thus has problems in safety at high temperature such as ignition, etc., caused by an internal short-circuit when internal heating occurs due to overcharge or overdischarge when applying to a lithium ion secondary battery.
Further, when the lithium secondary battery is initially charged, lithium ions from lithium metal oxide, which is a cathode, move to a carbon electrode, which is an anode, to thereby be intercalated into carbon. At this time, a surface of carbon particles, which are anode active materials, reacts with an electrolyte since lithium has high reactivity, and thus, a coating film called a solid electrolyte interface (SEI) film is formed on the surface of the anode.
Performance of the lithium secondary battery largely depends on a constitution of the organic electrolyte and the SEI film formed by the reaction between the organic electrolyte and the electrode.
That is, the formed SEI film suppresses a side reaction between a carbon material and an electrolyte solvent, for example, decomposition of the electrolyte on the surface of the carbon particles which are the anode, prevents collapse of the anode material due to co-intercalation of the electrolyte solvent into the anode material, and faithfully performs a role as a lithium ion tunnel according to the related art, thereby minimizing deterioration of battery performance.
Therefore, various studies have been attempted to develop a novel organic electrolyte including an additive so as to solve the above problems.
For example, Japanese Patent No. 2002-260725 discloses a non-aqueous lithium ion battery capable of preventing overcharge current and the thus-resulting thermal runaway phenomenon by using an aromatic compound such as biphenyl. In addition, U.S. Pat. No. 5,879,834 discloses a method of improving battery stability by adding a small amount of aromatic compounds such as biphenyl, 3-chlorothiophene, etc., to be electrochemically polymerized in an abnormal over-voltage state, thereby increasing internal resistance. However, in the case of using the additives such as biphenyl, etc., in a normal operating voltage, when relatively high voltage is locally generated, the additives are gradually decomposed in a charge and discharge process, or when the battery is discharged at high temperature for a long time, amounts of biphenyl, etc., are gradually reduced, and thus, after 300 cycles of the charge and discharge process, there are problems in that safety may not be guaranteed, storage characteristic is reduced, etc.
Therefore, studies into technology of improving stability at high temperature and low temperature while maintaining a high capacity retention ratio have been still demanded.